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The Physical Layer. The Physical Layer performs bit by bit transmission of the frames given to it by the Data Link Layer. The specifications of the Physical Layer include:Mechanical and electrical interfacesSockets and wires used to connect the host to the networkVoltage levels uses (e.g. -5V an
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1. CPSC441
2. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
3. Signal Transmission Electronic energy to send signals that communicate from one node to another
Two methods of transmitting data
Digital signaling
Analog signaling
4. Comparison of Digital and Analog
5. Bandwidth-Limited Signals
6. Bandwidth-Limited Signals
7. Digital Signaling Digital signal represents discrete state (on or off)
Practically instantaneous change
8. Digital SignalingCurrent State Encoding Data is encoding by the presence or absence of a signal
A positive voltage might represent a binary zero or binary one or visa versa
The current state indicates the value of the data
9. Digital Signaling Current State
10. Digital Signaling State Transition
11. Analog Signaling Signals represented by an electromagnetic wave
Signal is continuos and represents values in a range
Uses one or more of the characteristics of an analog wave to represent ones and zeros
12. Characteristics of an Analog Signal
13. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
16. Wire Propagation Effects Propagation Effects
Signal changes as it travels
Receiver may not be able to recognise it
17. Propagation Effects: Attenuation Attenuation: signal gets weaker as it propagates
Attenuation becomes greater with distance
May become too weak to recognise
18. Propagation Effects: Distortion Distortion: signal changes shape as it propagates
Adjacent bits may overlap
May make recognition impossible for receiver
19. Propagation Effects: Noise Noise: thermal energy in wire adds to signal
Noise floor is average noise energy
Random signal, so spikes sometimes occur
20. Propagation Effects: SNR Want a high Signal-to-Noise Ratio (SNR)
Signal strength divided by average noise strength
As SNR falls, errors increase
21. Propagation Effects: Interference Interference: energy from outside the wire
Adds to signal, like noise
Often intermittent, so hard to diagnose
22. Propagation Effects: Termination Interference can occur at cable terminator (connector, plug)
Often, multiple wires in a bundle
Each radiates some of its signal
Causes interference in nearby wires
Especially bad at termination, where wires are unwound and parallel
23. Bandwidth Capacity of a media to carry information
Total capacity may be divided into channels
A channel is a portion of the total bandwidth used for a specific purpose
24. Bandwidth Baseband
The total capacity of the media is used for one channel
Most LANs use baseband
Broadband
Divides the total bandwidth into many channels
Each channel can carry a different signal
Broadband carries many simultaneous transmissions
25. Analog versus Digital Digital
Is less error prone
Distortion of the signal between the source and destination is eliminated
Analog
Little control over the signal distortion
Old technology
26. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
27. Benefits of Digital Transmission Reliability
Can regenerate slightly damaged signals
There are only two states. Change to closest
E.g., if two states are voltages +10v (1) and -10v (0) and the signal is +8v, the signal is a 1
28. Benefits of Digital Transmission Error detection and correction
Can correct errors in transmission
Add a few bytes of error-checking information
Can ask for retransmission if an error is detected
29. Benefits of Digital Transmission Encryption
Encrypt (scramble) messages so that someone intercepting them cannot read them
30. Benefits of Digital Transmission Compression
Compress message before transmission
Decompress at other end
Compressed message places lighter load on transmission line, so less expensive to send
Not always used
31. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
32. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
33. Analog Signaling Amplitude Modulation (ASK)
34. Analog Signaling Frequency Modulation (FSK)
35. Analog Signaling Phase Shift Keying (FSK)
36. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
37. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
38. Modems Modulation demodulation
Used to connect a digital computer to an analog phone system
Can be installed internally a card inserted into the motherboard
Can be connected to the serial port (external modem)
39. Modems Transfer speeds
Bit rate BPS
Baud
Bandwidth
Compression
appears to increases speed by decreasing the number of bits sent (usually some data does not compress well)
sending and receiving modem must use same compression standard
40. Modems Error detection and correction
asynchronous modems use parity check
checksum counting the number of data words sent
41. Digital Modem Miss named
Used to connect to a digital telephone
ISDN (integrated Services Digital Network) is an example
Again do not connect digital modems to an analog phone
Higher quality lower errors
42. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
43. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
44. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
45. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
46. Network Media Types Types of Media
Cable (conducted media)
Coaxial
Twisted pair (UTP)
Shielded twisted pair (STP)
Fiber optic
Radiated
Infrared
Microwave
Radio
Satellite
47. Media Selection Criteria Cost
For actual media and connecting devices such as NICs hubs etc
Installation
Difficulty to work with media
Special tools, training
48. Media Selection Criteria Capacity
The amount of information that can be transmitted in a giving period of time
Measured as
Bits per second bps (preferred)
Baud (discrete signals per second)
Bandwidth (range of frequencies)
49. Media Selection Criteria Node Capacity
Number of network devices that can be connected to the media
Attenuation
Weakening of the signal over distance
50. Media Selection Criteria Electromagnetic Interference (EMI)
Distortion of signal caused by outside electromagnetic fields
Caused by large motors, proximity to power sources
Other noise sources
White (Gaussian) noise
Impulse noise
Crosstalk
Echo
51. Cable Media Unshielded Twisted Pair UTP
Shielded Twisted Pair STP
Coaxial
Fiber optic
52. Unshielded Twisted Pair (UTP) One or more pairs of twisted copper wires insulated and contained in a plastic sheath
Twisted to reduce crosstalk
53. Unshielded Twisted Pair (UTP) Categories
categories 1 and 2
voice grade
low data rates up to 4 Mbps
category 3
suitable for most LANs
up to 16 Mbps
category 4
up to 20 Mbps
54. Unshielded Twisted Pair (UTP) Categories
category 5
supports fast ethernet
more twists per foot
more stringent standards on connectors
Data grade UTP cable usually consists of either 4 or 8 wires, two or four pair
Uses RJ-45 telephone connector
55. Unshielded Twisted Pair (UTP)
56. Shielded Twisted Pair (STP) Same as UTP but with a aluminum/polyester shield
Connectors are more awkward to work with
Usually comes in pre made lengths
Different standards for IBM and Apple
57. Shielded Twisted Pair (STP)
58. Coaxial Cable Coax Two conductors sharing the same axis
A solid center wire surrounded insulation and a second conductor
59. Coaxial Cable
60. Coaxial Cable Coax Size of Coax
RG-8, RG-11
50 ohm Thick Ethernet
RG-58
50 ohm Thin Ethernet
RG-59
75 ohm Cable T.V.
RG-62
93 OHM ARCnet
61. Fiber Optic Cable Thin strand(s) of glass or plastic protected by a plastic sheath and strength wires or gel
Transmits laser (single mode) or LED (multi mode)
Single mode more expensive but can handle longer distances
62. Fiber Optics Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.
Light trapped by total internal reflection.
63. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
64. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
65. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
66. Fiber Optic Networks A fiber optic ring with active repeaters.
67. Fiber Optic NetworksStar connection
68. Fiber Cables A comparison of semiconductor diodes and LEDs as light sources.
69. Characteristics of Cable Media
70. Wireless Media Uses the earth’s atmosphere as a conducting media
Main types
radio wave
microwave (including satellite)
infrared
71. The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication.
72. Radio Transmission (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth.
(b) In the HF band, they bounce off the ionosphere.
73. Radio Wave Most radio frequencies are regulated
Must obtain a license from a regulatory board (CRTC, FCC)
A range of radio frequencies are unregulated
74. Radio Wave Low power single frequency
uses one frequency
limited range 20 to 30 meters
usually limited to short open environments
75. Radio Wave High power single frequency
long distance may use repeaters to increase distance
line of sight or bounced of the earth?s atmosphere
uses a single frequency
76. Radio Wave Spread Spectrum
Maintains security of the radio
transmission by:
Spreading the carrier signal frequency
Modulating the carrier frequency by a Pseudo Random signal
79. Radio Wave
80. Microwave Terrestrial
line of sight
use relay towers
uses license frequencies
81. Communication Satellites
82. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
83. Communication Satellites The principal satellite bands.
84. Communication Satellites VSATs using a hub.
85. Globalstar
86. Infrared Uses same technology as remotes for T.V.
signals can not penetrate objects
Can be point to point or broadcast
Point to point requires precise alignment of devices
Point less immune to eavesdropping
87. Multiplexing several lines (one for each device) enter a multiplexer (mux) at the host side
the host side mux combines all incoming signals
combined signals are transmitted to a mux on the receiving side
88. Types of Multiplexers Frequency Division Multiplexing (FDM)
Time Division Multiplexing (TDM)
Statistical Time Division Multiplexing (STDM)
89. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
90. Frequency division multiplexed circuit
91. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
92. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
93. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
94. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
95. Time Division Multiplexing
96. Statistical Time Division Multiplexing (STDM) allows connection of more nodes to the circuit than the capacity of the circuit
works on the premise that not all nodes will transmit at full capacity at all times
must transmit a terminal identification
may require storage
97. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
98. WAN Transmission Media Public Switched Telephone Network
High speed, High bandwidth dedicated leased circuits
High speed fiber optic cable
Microwave transmission links
Satellite links
99. Services provided by PSTN Voice – Plain Old Telephone Service (POTS)
Based on the Voice Channel (3600 Hz)
Data transmission services
consist of services such as :
switched 56
X.25
T1, T3 circuits
Frame relay
ISDN
ATM
100. Switching Switching send data across different routes
Three types of switching
Circuit switching
Message switching
Packet switching
101. Switching Circuit Switching: A connection (electrical, optical, radio) is established from the caller phone to the callee phone. This happens BEFORE any data is sent.
Message Switching: The connection is determined only when there is actual data (a message) ready to be sent. The whole message is re-collected at each switch and then forwarded on to the next switch. This method is called store-and-forward. This method may tie up routers for long periods of time - not good for interactive traffic.
Packet Switching: Divides the message up into blocks (packets). Therefore packets use the transmission lines for only a short time period - allows for interactive traffic.
102. Circuit Switching Connects the sender and receiver by a single physical path for the duration of the session
PSTN uses circuit switching
Before transmission a dedicated circuit must be established
103. Circuit Switching Advantages
guaranteed data rate
once connected no channel access delay
Disadvantages
inefficient use of the transmission media (idle time)
long connection delays (first time)
104. Message Switching Each message is treated as an independent unit
has its own source and destination address
Each is transmitted from device to device
Each intermediate device stores the message until the next device is ready
store and forward
105. Message Switching Route messages along varying paths for more efficiency
Switching devices are often PCs with special software
106. Message Switching Advantages
efficient traffic management
reduces network congestion
efficient use of network media
messages can be sent when receiver down
Disadvantages
delay of storing and forwarding
costly intermediate storage
107. Packet Switching Packet switching breaks messages into packets
Packets travel different routes (independent routing)
Each packet has its own header information
Packets small enough to be stored in RAM thus quicker than message switching
108. Packet Switching Advantages
improves the use of bandwidth over circuit switching
can adjust routes to reflect network conditions
shorter transmission delays than message switching (stored in RAM)
less disk space
smaller packets to retransmit
109. Packet Switching Disadvantages
More RAM
More complex protocols
more processing power for switching device
Greater number of packets greater chance for packet loss or damage
110. Packet Switching Two methods of packet switching
Datagram packet switching
Virtual circuit packet switching
111. Datagram Packet Switching
Message divided into a stream of packets
Each packet has it’s own control instructions
Switching devices route each packet independently
112. Datagram Packet Switching Switching devices can route packets around busy network links
Require sequence numbers to reassemble
Small packet size facilitates retransmission due to errors
113. Virtual Circuit Packet Switching Similar to circuit switching
Before transmission of the sending and receiving device agree on:
maximum message size
network path
establish a logical connection (virtual circuit)
All packets travel on the same virtual path
114. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
115. (a) circuit switching(b) message switching(c) packet switching
116. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
117. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
118. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
119. B-ISDN (Broadband Integrated Services Digital Network) Broadband transmission ? A type of data transmission in which a single medium (wire) can carry several channels at once (ex. Cable TV).
Baseband transmission ? one signal at a time (most communication between computers).
B-ISDN ? will offer video on demand, live television from many sources, full motion multimedia email, CD-quality music, LAN interconnection, high-speed data transport for science and industry and many more, ALL over the telephone line.
A digital virtual circuit capable of 155 Mbps
Underlying technology that makes B-ISDN possible ? ATM (Asynchronous Transfer Mode)
120. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.
121. Why all the interest in ATM? These days it is more common for companies to want to interconnect networks. There is one international standard for ATM networks and so interconnection is easier.
ATM can be used to provide both LAN and WAN networks.
ATM can be made to behave like other standard networks and so you do not have to throw away all your old equipment.
ATM provides a high speed network.
122. ATM Technology ATM is based on Cell Relay technology. This uses virtual circuits to carry small packets (just 53 bytes long) over a predetermined path through the network.
Section of the path can be shared by other virtual circuits, thus ensuring that the network is used more efficiently than in the case of circuit switching.
Little error checking is performed by the network (this keeps overheads down). Instead, the transmitting and receiving hosts are responsible for error checking.
123. Establishing a Connection When information needs to be communicated, the sender NEGOTIATES a "requested path" with the network for a connection to the destination.
When setting up this connection, the sender specifies the type, speed and other attributes of the call, which determine the end-to-end quality of service.
The network then determines a path through the network, and sets up a virtual circuit along this path.
An analogy for this is sending mail. One can choose to send 1st class, overnight, 2 day delivery, etc. and can ask for certified mail.
124. ATM Cells
125. ATM Cell Transmission
126. ATM Speed ATM can deliver data at rates of either 155.52 Mbps or 622.08 Mbps and higher data rates are likely to follow.
ATM can operate at these high speed because specialised switching mechanisms have been developed that can switch the short 53 byte cells very quickly through the network.
ATM can work over a variety of media. Coaxial cable and optical fibre are the most commonly used.
ATM technology is currently being used to develop the next generation of high-bandwidth telephone systems.
127. In order to implement this algorithm we had to address these six issues.
In order to implement this algorithm we had to address these six issues.